Xbench 1.3での、2TB HDD (3Gbps接続）と 850 PRO 2TB (6Gbps接続)とのIO性能の比較結果
Random Read/Write は SSDはHDDに比べて圧倒的に高速ですね。

Mac Pro (Mid 2010) 3.33 GHz 6-Core Intel Xeon/ 24 GB 1333 MHz DDR3 ECC
OS X ElCapitan version 10.11.1（15B42）

Hitachi　3.5” 2TB HDD (3Gbps)　(Mac Proに付いてきたHDD,3.5” Bay)

Results 61.25
System Info
Xbench Version 1.3
System Version 10.11.1 (15B42)
Physical RAM 24576 MB
Model MacPro5,1
Drive Type Hitachi HDS722020ALA330
Disk Test 61.25
Sequential 139.81
Uncached Write 235.63 144.67 MB/sec [4K blocks]
Uncached Write 202.63 114.65 MB/sec [256K blocks]
Uncached Read 66.62 19.50 MB/sec [4K blocks]
Uncached Read 226.18 113.68 MB/sec [256K blocks]
Random 39.21
Uncached Write 12.62 1.34 MB/sec [4K blocks]
Uncached Write 135.32 43.32 MB/sec [256K blocks]
Uncached Read 103.68 0.73 MB/sec [4K blocks]
Uncached Read 174.67 32.41 MB/sec [256K blocks]